Electrical feedthroughs in a substrate or in a subarea of a substrate, such as a wafer, for example, exist in a wide variety of embodiments. The aim is always to achieve as small as possible a feedthrough at a low electrical volume resistance. To achieve that, frequently a narrow through-hole with almost vertical walls is produced in the substrate concerned, the wall is electrically insulated, and then the through-hole is completely or partially filled with a metal or a metal alloy in order to obtain the desired low volume resistance.
Depending on the application, that known approach is subject to limitations. On the one hand, there are applications in which the presence of metal produces interference. As an example of numerous MEMS applications, the micromechanical pressure sensor may be mentioned here.
FIG. 3 shows a schematic cross-sectional illustration to explain the set of problems underlying the present invention, with reference to an example of a substrate having an electrical feedthrough and a pressure sensor.
In FIG. 3, reference numeral 2 denotes a silicon semiconductor substrate. A first region 1 having an electrical feedthrough 6a and a second region 11 having a micromechanical component in the form of a pressure sensor are provided in silicon semiconductor substrate 2. Feedthrough 6a is connected to a first electrical contact terminal DK1 of pressure sensor 11a via a strip conductor 15a on front side V of substrate 2. Pressure sensor 11 has a diaphragm 3 which is provided over a cavity 3a. A piezoresistive resistor 4 and an isolation well 4a situated therebeneath are diffused into diaphragm 3. First electrical contact terminal DK1 and, in addition, a second electrical contact terminal DK2 contact piezoresistive resistor 4 in such a way that the piezoelectric resistance is detectable between them.
A first insulating layer I1 is provided between electrical metal strip conductor 15a and front side V of substrate 2. A second insulating layer I2 is provided between a back-side electrical metal strip conductor 15b and back side R of substrate 2. Insulating layers I1 and I2 may, for example, be oxide layers. Feedthrough 6a connects front-side strip conductor 15a to back-side strip conductor 15b. A wall insulating layer 7a, which is likewise made of oxide, for example, isolates feedthrough 6a from surrounding substrate 2. Lastly, reference numeral 9 denotes what is referred to as a seed layer for applying the metal of feedthrough 6a, which at the same time may be used as a diffusion barrier.
In such classical micromechanical pressure sensors 11, deformation of silicon diaphragm 3, which is disposed on silicon substrate 2, is measured by way of the piezoresistive resistance. When the pressure changes, the deformation of diaphragm 3, and hence the resistance signal of piezoresistive resistor 4, changes. Owing to the differing material parameters of silicon and metal, even the narrow metal strip conductors 15a situated on the surface and in the vicinity of diaphragm 3 cause voltages which are transmitted via substrate 2 to diaphragm 3. It is possible with some effort to compensate for the temperature-dependent component of the voltages. However, the inelastic properties of many metals also cause hysteresis in the characteristic curve of the pressure sensor. It is not possible to compensate for that effect. When metallic regions are provided not only at the surface but also at a depth within substrate 2, distinctly greater adverse effects on voltage-sensitive components, such as pressure sensors, for example, are also expected.
On the other hand, there are a number of applications in which primarily also high voltages or also only high voltage peaks (ESD, for example) are to be conducted through a substrate or a subarea of the substrate via an electrical feedthrough. This proves to be difficult with the approach described above. Isolation of the etched through-holes is usually achieved by oxide deposition. The achievable oxide thicknesses are greatly limited by the process control and the specific geometry. Accordingly, the maximum dielectric strength is also greatly limited. In addition, the surface of the through-holes, which is formed using a trench etching process or a laser process, is rather rough. That roughness causes electric field peaks which likewise reduce the dielectric strength.
Alternative approaches that manage without metals are not usable in many applications, since only with metals is it possible to achieve the extremely low volume resistances that are often necessary.
FIG. 4 shows a schematic cross-sectional illustration to explain the set of problems underlying the present invention, with reference to a substrate having an electrical feedthrough and a pressure sensor as known from German patent document DE 10 2010 039 330 A1.
Substrate 2 shown in FIG. 4 has an electrical feedthrough 6 running through substrate 2 from its front side V to its back side R. An annular isolation trench IG in substrate 2 surrounds electrical feedthrough 6 and is closed off by first insulating layer I1 on front side (V) and by second insulating layer I2 and a closing layer 18 on back side R of substrate 2. A thin liner-shaped insulating layer 18′ is formed on the walls of annular isolation trench IG. Annular isolation trench IG may be filled or unfilled (as illustrated). An annular substrate region 2a surrounds electrical feedthrough 6. An electrical contact terminal DK3 to electrical feedthrough 6 is formed on back side R of substrate 2. An electrical strip conductor 5 is formed on front side V of substrate 2 between a contact ring 5a, situated on electrical feedthrough 6, and pressure sensor 11a. Thereover, a dielectric protection layer 16, for example made of nitride, is formed.
The subject matter of German Patent document DE 10 2010 039 330 permits the production of metallic feedthroughs through a substrate, with a high dielectric strength and voltage decoupling between the metalized region and the substrate being possible.
The method from German patent document DE 10 2010 039 330 is, however, relatively laborious and expensive. In addition, the combination disclosed therein of metallic punch and separate isolation ring permits only relatively large feedthroughs to be produced.